It is common practice in various petroleum and chemical operations to heat fluids by using multipass heater systems. As used herein a "pass" is a fluid path through a furnace coil or a heat exchanger coil. A plurality of parallel passes for heating the fluid can be formed by multiple coils disposed in a furnace or by parallel arrangement of heat exchangers or by a combination of multiple furnace coils and heat exchanger passes. The inlet flow of fluid is divided to flow through the separate passes and the heated fluid in the outlet flow from the separate passes is recombined.
It is generally desirable to equalize the outlet temperatures of the fluid flowing in the multiple heat transfer passes so as to substantially maximize the transfer of heat to the fluid flowing through the multiple passes. This may be accomplished by manipulating the flow of fluid in each pass in response to a temperature measurement for each pass so as to maintain the outlet temperatures of all passes substantially equal.
Heater systems of this type are often used for preheating crude oil for introduction into a distillation tower. However, if the crude oil feed rate varies, as will usually be the case, control based on temperature measurements only will not be able to respond to the changing crude oil feed rate. The temperature may change but, since the control is seeking to maintain temperature equality, there may be no change in the position of the control valves in the multiple passes. This may result in an undesired total flow to the crude tower, an undesired liquid level in the crude tower, or an undesired gas/liquid ratio in the tower.
In crude oil fractional distillation it is extremely desirable to utilize various process streams which are associated with the fractional distillation process and which require cooling, to accomplish preheating of the crude oil. Typically heat which can be recovered from the various process streams in liquid-to-liquid heat exchangers is supplemented by a furnace or a plurality of furnaces which supply additional heat to the crude oil feed as required. Therefore a crude oil distillation preheat system can comprise a plurality of parallel passes where a portion of the passes are through furnace coils and the remaining passes are through liquid-to-liquid heat exchangers. Further it is noted that the physical size and the maximum flow rate possible through each of the passes is not necessarily equal.
During the normal course of operation it often happens that flow through one or more of the passes is limited because, for example, the temperature required for equalizing the pass exceeds a maximum temperature limit for the furnace or heat exchanger associated with that pass. Other factors such as maintenance requirements can also restrict operation for one or more of the multiple parallel passes such that manipulating the flow in a particular pass to equalize its temperature with the other passes is not desirable.